Specimens for use in a transmission electron microscope (TEM) are prepared so that they are sufficiently thin to allow incident electrons to be transmitted through the specimen so that they can form a high resolution image. The TEM lens conditions can also be modified so that a diffraction pattern can be obtained. In order to achieve a condition wherein the specimen, which is typically less than 200 nm thick in the direction of the electron beam, is electron-transparent, the incident electrons are typically accelerated to energies between 80 keV and 400 keV.
Often such instruments can also be run in a scanning TEM mode (STEM), wherein a finely-focussed spot is deflected in a raster pattern over the specimen and a signal from transmitted or secondary electrons is recorded at each position so that the image is built up in a serial fashion. It is useful to know the thickness of a specimen because this can be used in the calculation of its other properties, such as point defect or dislocation density. Several techniques exist for measuring thickness in the TEM, which are based on measurement of transmitted or scattered electron signals or on analysis of electron diffraction patterns.
In the scanning electron microscope (SEM) or electron probe microanalyser (EPMA), the incident electron beam energy is typically below 40 keV and the image is formed by secondary or backscattered electrons that emerge from the surface of the specimen that is exposed to the incident electron beam. In such instruments it is desirable to measure thicknesses in certain types of specimens. A known method for measuring the thickness and composition of a thin layer on a substrate in the SEM or EPMA involves measuring the intensities of characteristic X-ray emissions for each chemical element. The specimen is struck by an electron beam with sufficient energy to penetrate through the layer to the substrate and the X-ray intensity for each element is divided by to the intensity produced when a pure bulk element standard is struck by the same beam, thereby obtaining a “k-ratio” for each element. The k ratios for a thin film will be less than unity. Suitable corrections to account for electron beam scattering by the layer and the substrate can be used to deduce the thickness and composition of the layer (e.g. J. L. Pouchou. “X-ray microanalysis of stratified specimens”, Analytica Chimica Acta, 283 (1993), 81-97).
It has also been recognised that an unsupported layer or film can be analysed in the SEM or EPMA by the same approach used for analysing a layer on a substrate (Dijkstra et al, Microchimica Acta 114/115, 277-284, 1994). In this case, the unknown thin film is supported on the sort of grid typically used in the TEM so that at the highest available beam energy the incident beam will penetrate through the film and emerge into a vacuum rather than a substrate. The k-ratios for the characteristic element emissions are again measured by comparing the X-ray intensity relative to the intensity obtained when a bulk pure element is struck by the same incident beam. The thickness and composition of the film is deduced by modifying the applied corrections to take account of the lack of any scattering from the substrate. If the program cannot be modified so as to model the absence of a substrate, a suitable modification can be achieved by assuming that in place of a vacuum, there is a substrate of very low atomic number material, such as Beryllium, which would not produce significant back scattering.
As noted by Dijkstra et al, the same SEM or EPMA approach could in principle be used in the TEM. However, this is accompanied by a major disadvantage in that the X-ray yield from a bulk specimen increases strongly with incident electron energy. Therefore, when a pure bulk specimen is exposed to the same electron beam as is used to collect data from the thin specimen, the X-ray intensity, even at minimum beam current, causes an excessive count rate. Therefore, it is not practical to measure the X-ray intensities produced in the specimen and a pure bulk element standard using the same beam current. Boon (G. Boon, Thesis 2000 ISBN 90-386-2781-5) recognised this problem and devised a special beam current meter that could operate reliably over 3 to 4 decades. Boon recorded X ray data from the bulk standards at a much lower beam current than for the specimen to avoid overload of the X-ray spectrometer. The accurate beam current measurement enabled the correction of the X-ray intensities from specimen and the reference standard so that they corresponded to the same incident beam current and thus the k-ratios could be determined.
A further issue recognised by Dijkstra et al is that the penetration depth also increases strongly with incident electron energy, such that in the TEM, X-rays are generated at greater depths within the pure bulk element standard and are more likely to be absorbed as they emerge towards the detector. The required correction for absorption is therefore much higher at high electron energies and is particularly high for characteristic X-rays with low energy. In many TEMs, the X-ray detector is mounted so that it only detects X-rays emerging from the specimen surface at a shallow “take-off-angle”. Such X-rays experience high absorption as they emerge from depth in the specimen. In order to reduce this absorption, the specimen surface can be tilted towards the X-ray detector, however this alters the penetration of incident electrons and complicates the correction calculations.
Whereas Boon had some success in applying X-ray corrections for a bulk pure sample with surface normal to the electron beam, he recognised that improvements were needed for analysis of light elements (low characteristic X-ray energy), and for tilted specimens. In addition, the need to measure beam currents and X-ray intensities from bulk standards for every element present renders the analysis of materials composed of several elements difficult.
There remains a strong need for a practical method of measuring the thickness of thin samples for use in electron microscopy.